1. Field of the Invention
This invention relates to a coated cemented carbide having a very high toughness, used for cutting tools, etc. and more particularly, it is concerned with a high efficiency cutting tool consisting of a cemented carbide substrate coated with a vapor deposited thin film such as of titanium carbide, having jointly a high toughness of the substrate and high wear resistance of the surface coating.
2. Description of the Prior Art
Recently, N/C machines have been introduced into the field of cutting processing to markedly advance the so-called factory automation. In such a case, the reliability of cutting tools is very important and it is thus required to develop a cutting tool having a higher toughness than those of the prior art.
In order to satisfy this requirement, there have been proposed cemented carbide alloys in which only the surface layer consists of WC-Co (Japanese Patent Laid-Open Publication Nos. 159299/1977 and 194239/1982), methods comprising enriching the surface of an alloy with Co (Japanese Patent Laid-Open Publication Nos. 105628/1987, 187678/1985 and 194239/1982, i.e. U.S. Pat. No. 4,610,931) and a method comprising allowing free carbon to exist in an alloy so as to prevent formation of a decarburized layer just under a coating layer (Japanese Patent Laid-Open Publication No. 155190/1977).
However, the cemented carbide alloy having a WC-Co layer on only the surface or having a Co-enriched layer on the surface can exhibit improved toughness, but meets with a problem on wear resistance. At a higher cutting speed, in particular, the alloy having a Co-enriched layer cannot sometimes be put to practical use because of the higher wearing speed of a rake face. In the case of the alloy containing free carbon (FC), the toughness is improved with the increase of the amount of carbon, but if it exceeds 0.2% by weight, the alloy becomes agglomerative to lower the strength itself of the alloy.